1. Field of the Invention
The present invention concerns a method to generate projection images of the inside of an examination subject. The invention furthermore concerns a device to generate projection images of the inside of an examination subject and a control device for such a device.
2. Description of the Prior Art
A method of the general type described above is typically applied in a computed tomography device (CT device). Such a CT device 1 is shown in FIG. 1. Volume exposures and/or slice images of the inside of an examination subject 2 can be generated in a non-invasive manner with the CT device 1. In order to enable a complete reconstruction of the three-dimensional anatomy of the examination subject 2, projections (projection data sets) Pi of the examination subject 2 must initially be made from different viewing directions. The spatial exposures and/or slice images are then reconstructed from the projections Pi.
For the acquisition of the projections Pi, the examination subject 2 is positioned in an examination space 3 and an x-ray source 4 and a detector system 5 are rotated around the examination subject 2 in a rotation direction R. The detector system 5 is arranged diametrical to the rotation center relative to the x-ray source 4, so the rotation center coincides with the center of the examination space 3. The detector system 5 has a number of discrete detectors distributed circumferentially with alignment on the x-ray source 4. These detectors are known as channels 6i to those skilled in the art.
The x-ray source 4 serves to generate the x-rays. A diaphragm device 7 (the position of which is indicated only schematically in FIG. 1) is located between the focus 10 of the x-rays and the examination subject 2 at the x-ray source 4. With the diaphragm device 7, the x-rays are delimited in the form of a ray beam 8, for example in the form of a fan or cone beam. The diaphragm device 7 is most often realized as a slit diaphragm and, by its slit dimensions, demarcates a fan angle θ of the ray beam 8, with the x-ray radiation that strikes the slit diaphragm outside of the slit dimension being masked (absorbed). In order to optimally acquire data for the entire examination subject 2 with each projection Pi, the fan angle θ must be made relatively large; a value of 50° is typical for normal whole body exposures, for example. The circumferential extent of the detector system 5 is also accordingly large, with the individual channels 6i being located at discrete positions along this circumferential extent, as is schematically indicated in FIG. 1. The same essentially also applies in the use of detectors known as multi-line detectors, in which multiple lines of channels 6i arranged adjacent to one another are fashioned transverse to the circumferential dimension, i.e. parallel to the rotation axis of the system. The ray beam—in the present case a conical fan or cone beam—is then dimensioned such that these adjacent lines are struck by x-rays attenuated y the subject 2 as well.
In order to be able to meaningful reasonable images, projection data from all directions should be present for each channel 6i. It should be noted that consideration of an examination subject from opposite directions delivers no additional information. Therefore, the combination of the x-ray source 4, the diaphragm device 7 and the detector system 5 that is mounted on a gantry 9 must be rotated by at least 180°, plus the fan angle θ of approximately 50° plus a transition angle of approximately 30°. Even though the minimum total angle necessary for the image reconstruction therefore amounts to approximately 260°, this “partial revolution”, as it is called, is designated as a “180° scan”. This situation is schematically depicted in FIG. 2, wherein not the full rotation of the stated 260°, but rather only a segment of this that is necessary for further explanation of the function, is shown. Located in the circumferential direction is a focus 10 of the ray beam 8 (ray beam 8 represented with solid lines), initially at the position of approximately φ=−25° (ray beam 8 represented with broken lines). The known diaphragm used to delimit the ray beam 8 has a diaphragm aperture 1 that is independent of the rotation angle φ. In addition to a number of functions, among other things the operation of the x-ray source 4 and the rotation of the gantry 9 are controlled by a control device 32 (shown in FIG. 1) that for this purpose has an x-ray source control module 32A and a gantry control module 32B.
A contribution of the individual channels 6i for the reconstruction of images of the inside of the examination subject 2—and therefore also their suitability for use in this reconstruction—is illustrated in FIG. 3 in the form of a sinogram. The channels 6i through 6n (generally 6i) are plotted on the x-axis and the projections Pi are plotted on the y-axis, for example in time intervals of 1 ms during the rotation. The upper triangular region 12 and the lower triangular region 13 cannot be used for the reconstruction because not all channels 6i deliver a contribution at those regions. Only the rectangular region 14 is usable for reconstruction.
It should be noted that the examination subject 2 is continuously exposed while the gantry 9 rotates, but only a portion of the applied dose can actually be used for image reconstruction. The unusable dose is predominantly applied at the beginning and end of the partial revolution, as is apparent from FIG. 3. The proportion of the unused dose increases with the size of the fan angle θ and decreases with increasing rotation angle φ. The relative proportion of the unused dose is calculated as the quotient of the fan angle θ and the rotation angle φ. The effect is greatest in the 180° scan and amounts to approximately 19% of the total dose.
This disadvantage exists only for the single source CT device 1 but also described above, in a further type of device known as a dual source CT device, in which two ray beams 8 are used that are offset by 90° from one another. The two ray beams are rotated by 90°. Such a partial revolution is accordingly also designated as a 90° scan. The sinogram of such a dual source CT device is shown in FIG. 23. The aforesaid triangle regions 12 and 13 that cannot be used for reconstruction exist in this case as well. In contrast to the sinogram of the single source CT device 1, however, three usable rectangular regions 14 now exist between these outer triangular regions 12 and 13, with the middle rectangular region 14 being acquired by contributions of the two ray beams 8 that supplement one another.
In summary, with regard to the unusable triangular regions 12 and 13 it should be noted that it is desirable (in particular for medical applications) to keep the dose as low as possible in order to expose the examination subject to an optimally low radiation dose.